The present invention relates to an active matrix liquid crystal display (LCD), and more particularly to a video signal recording/regenerating device, such as an electronic still camera and the like, employing a liquid crystal display as a monitor.
Recently, liquid crystal displays are being used as monitors in video signal recording/regenerating devices, making for a compact and relatively small power consumption device. Among the various types of liquid crystal displays, active matrix type displays having display elements and switching elements are becoming more and more prevalent, because displays employing other methods such as a direct multiplexing method are hardly capable of providing high-density display. In the active matrix type display, display elements are selectively driven by selectively turning ON corresponding switching elements.
FIG. 1 is a circuit diagram illustrating an example of a conventional thin film transistor (TFT) active matrix LCD.
As shown in FIG. 1, a gate drive circuit 1 supplies switching pulses to a predetermined number of gate buses 2, whereas a source drive circuit 3 applies video signals to a predetermined number of corresponding source buses 4.
At each position of the matrix a thin film transistor (TFT) 5 functions as a switching element. A capacitor 6 accumulates the signal connected to each drain of the TFT's 5, and a display element 7, having transparent electrodes and liquid crystal layers respectively, are disposed at each position.
The operation will now be described with reference to a timing chart of FIG. 2.
The gate drive circuit 1 supplies switching pulses to the corresponding gate buses 2 being synchronized with horizontal synchronous signals included in the video signals.
Suppose a switching pulse (FIG. 2(a)) is supplied to a gate bus 2 of the i-th line with the predetermined timing synchronized with the horizontal synchronous signal, and a switching pulse (FIG. 2(b)) is supplied to the gate bus 2 of the (i+1)-th line as with the timing of the horizontal synchronous signal approximately one field later in the case of interlaced scanning on one field basis. Note that in the case of line sequential scanning, the interval between adjacent switching pulses of the ith and the (i+1)th gate buses respectively equals one horizontal scanning period. The TFT's 5 connected to the gate bus 2 to which the switching pulse has been supplied are thus turned ON.
The source drive circuit 3 divides the received video signal into a multiplicity signals corresponding to a number of horizontal scanning lines these signals are synchronized with the horizontal synchronous signal included in the video signal. These video signals are accumulated in each of the capacitors 6 via the source and drain of the each TFT 5 and delivered to each display element 7 for display.
The signals accumulated in the capacitors 6 are held until a switching pulse is again supplied, after a lapse of time corresponding to the period of one frame. It should be noted that if the capacity of the liquid crystal and the resistance ratio R.sub.off /R.sub.on (wherein R.sub.off is the internal resistance of the TFT 5 when it is in an OFF condition and R.sub.on is the internal resistance when it is in an ON condition) of the TFT 5 are sufficiently larger, the capacitor 6 can be omitted.
When the switching pulse is supplied to the gate bus 2 of the i-th line, the video signal (FIG. 2(c)) applied to the source bus 4 of the j-th column, is sampled and held in capacitor 6 as shown in FIG. 2(d).
If the potential shown in FIG. 2(e) has been supplied to the reference terminal of the capacitor 6, the voltage shown in FIG. 2(f) is applied to the display element 7.
It should be noted that since the charges accumulated by the capacitors 6 are in accordance with the preceding input image signal (FIG. 2(f)), the succeeding image signal is applied to each display elements 7 in reversed polarity. That is, the image signal is applied to each source bus 4 through the source drive circuit 3 as an alternating current. An interval between the image signals applied to each of display elements 7 corresponds to the period for one frame in an interlaced scanning method; or one field in a line sequential scanning method.
In this way, a video signal corresponding to each pixel is supplied so that a whole image is displayed.
In the case of an electronic still camera or the like, for instance, such a liquid crystal display can be used to confirm an image to be photographed to view a recording of a video signal of the image on a magnetic disk.
Moreover, the liquid crystal display can be used to monitor the image by regenerating the video signal from the magnetic disk.
As set forth above, when a video signal is supplied to a conventional liquid crystal display, the display functions to show an image for only a period of time that corresponds to one frame. This permits the display to indicate multiple frames, thus simulating animation (or motion) of an image. Accordingly, in order for a conventional liquid crystal display device to display a still image, a frame member (or field memory) has to be used to store a video signal equivalent to one frame (or field) and then apply the video signal to the liquid crystal display for display by repeatedly reading the stored signals or repeatedly regenerating the signals recorded on the magnetic disk.
As a result, expensive memory is required to regenerate a still image and the disadvantage is that the generation of the still image tends to become costly.
To obtain the still image by regenerating the video signal from the magnetic disk is also disadvantageous in that a battery, a magnetic head or disk, or the like is quickly consumed.